Gravitational waves: what breakthroughs can we expect next?

The gravitational wave detection by Ligo opens up the possibility of answering questions about other phenomena, including cosmic strings and dark energy

With the first success of Ligo (Laser Interferometer Gravitational-wave Observatory), astrophysicists and cosmologists have a new kind of telescope. What could they hope to see with this new instrument and its successors?

The answers could hardly be more explosive. The instrument that has just identified two black holes in a high-speed dance of death more than a billion light years away could be used to observe phenomena that currently only exist in theory – cosmic strings. Theoretically these strings are many light years in length, moving at the speed of light, thinner than an atom but so massive just an inch would weigh 10 million billion tons.

If two strings become entangled in distant space, a gravity wave telescope should spot it. And new telescopes of any kind have knack of identifying the unexpected. “Serendipity still rules astronomy, since we are very much a discovery-led science. Interestingly the source they discovered is not what one would have predicted, so that is a good start”, said Gerry Gilmore, professor of experimental philosophy of the Institute of Astronomy at Cambridge. He was prepared to speculate: “Maybe string-like defects thrashing around, left from Big Bang strings stretched by early inflation. Maybe higher dimensions popping in and out of existence”.

Such instruments could certainly detect collisions between neutron stars, in which the atomic structure is so densely packed that a matchbox full of neutron star would weight as much as Manhattan or Manchester. They could see neutron stars in collision, or observe a neutron star being ripped apart by a black hole, in each case with a tremendous release of energy in the shape of warps and distortions in spacetime that would ripple to the edge of the universe.

The telescope could ultimately be used to answer questions about dark energy. This is a mysterious force in space itself, which is sometimes called antigravity, and which is causing the distant universe to accelerate. Identified only in 1998, it accounts for more than two thirds of the mass of the cosmos.

“We really don’t know what dark energy is”, said Prof Sathyaprakash. “But what we can do is map the geometry of the universe by using black holes and neutron stars as distance markers. They can give us very precise distance. If we do this, we should be able to get a precise geometry of the universe and in the long run – it is not going to be easy – but in 10 or 15 years we will be able to figure out what is the characteristic of this dark energy”.